There are to date eight recognized human papillomaviruses (HPV) and five bovine papillomaviruses (BPV). Each of the human papillomaviruses and bovine papillomaviruses are associated with distinct clinical entities including common warts, condyloma accuminata, and laryngeal papillomatosis in humans, and esophageal papillomatosis and cutaneous fibropapillomas in cattle. To date none of the papillomaviruses has been successfully propagated in tissue culture and consequently little is known of their molecular biology. A subset of papillomaviruses, however, are associated with carcinomas in their natural hosts. These include two of the human papillomaviruses: type 6, which has an epidemiological association with carcinomas arising in condyloma accuminata; and the type 5 human papillomavirus, which is associated with squamous cell carcinomas in patients which epidermodysplasia verruciformis. In cattle the esophageal papillomatosis is associated with alimentary tract carcinomas in those cattle which feed on bracken fern. Bovine fibropapillomas are caused by a virus (BPV-1 or BPV-2) which are capable of inducing tumors in hamsters as well as transforming certain mouse cells in tissue culture.